Question 80747 in Docker Question posted in
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My changes were lost in new Docker container

Steps to reproduce:

  1. Download and run postgres:9.6.24:

    docker run --name my_container --restart=always -d -p 127.0.0.1:5432:5432 -e POSTGRES_PASSWORD=pgmypass postgres:9.6.24

    Here result:

    CONTAINER ID   IMAGE             COMMAND                  CREATED          STATUS          PORTS                      NAMES
879883bfc84a   postgres:9.6.24   "docker-entrypoint.s…"   26 seconds ago   Up 25 seconds   127.0.0.1:5432->5432/tcp   my_container

    OK.

  2. Open file inside container /var/lib/postgresql/data/pg_hba.conf

     docker exec -it my_container bash
   root@879883bfc84a:/# cat /var/lib/postgresql/data/pg_hba.conf

   IPv4 local connections:
   host    all             all             127.0.0.1/32            trust

  3. Replace file /var/lib/postgresql/data/pg_hba.conf inside container by my file. Copy and overwrite my file from host to container:

    tar --overwrite -c pg_hba.conf | docker exec -i my_container /bin/tar -C /var/lib/postgresql/data/ -x

  4. Make sure the file has been modified. Go inside container and open changed file

       docker exec -it my_container bash
       root@879883bfc84a:/# cat /var/lib/postgresql/data/pg_hba.conf
       IPv4 local connections:
       host    all             all            0.0.0.0/0                trust

    As you can see the content of file was changed.

  5. Create new image from container

    docker commit my_container

    See result:

    docker images
REPOSITORY   TAG       IMAGE ID       CREATED         SIZE
<none>       <none>    ee57ad4bc6b4   3 seconds ago   200MB 
postgres     9.6.24    027ccf656dc1   12 months ago   200MB

    Now tag my new image

    docker tag ee57ad4bc6b4 my_new_image:1.0.0

    See reult:

    docker images
REPOSITORY         TAG       IMAGE ID       CREATED              SIZE
my_new_image       1.0.0     ee57ad4bc6b4   About a minute ago   200MB
postgres           9.6.24    027ccf656dc1   12 months ago        200MB

    OK.

  6. Stop and delete old continer:

    docker stop my_continer
docker rm my_container

    See result:

    docker ps -a
   CONTAINER ID   IMAGE     COMMAND   CREATED   STATUS    PORTS     NAMES

    As you can see not exit any container. OK.

  7. Create new continer from new image

    docker run --name my_new_container_test --restart=always -d -p 127.0.0.1:5432:5432 -e POSTGRES_PASSWORD=pg1210 my_new_image:1.0.0

    See result:

    docker ps

CONTAINER ID   IMAGE                    COMMAND                  CREATED         STATUS         PORTS                      NAMES
3a965dbbd991   my_new_image:1.0.0   "docker-entrypoint.s…"   7 seconds ago   Up 6 seconds   127.0.0.1:5432->5432/tcp   my_new_container

  8. Open file inside container /var/lib/postgresql/data/pg_hba.conf

    docker exec -it my_new_container bash
   root@879883bfc84a:/# cat /var/lib/postgresql/data/pg_hba.conf

   IPv4 local connections:
   host    all             all             127.0.0.1/32            trust

    As you can see my change in files are lost. The content of file is original. Not my changes.

P.S. This problem is only with file pg_hba.config. E.g if I created in the container the folder and file: /Downaloads/myfile.txt then this file not lost in the my container "my_new_container".

Tags:

2

Answers


  1. 0

    Docker Container is a readyonly entity, which means if you will create a file into the container, remove it and re-create it (The container), the file is not supposed to be there.

    what you want to do is one of two things,

    1. Map your container to a local directory (volume)
    2. Create a docker file based on the postgres image, and generate this modifications in a script, that your dockerfile reads.

    docker volume usages

    Dockerfile Reference

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  2. 0

    Editing files inside container with docker exec, in general, will in fact cause you to lose work. You mention docker commit but that’s almost never a best practice. (If this was successful, but then you discovered PostgreSQL 9.6.24 exactly had some critical bug and you must upgrade, could you recreate the exact some image?)

    In the case of the postgres image, the files in /var/lib/postgresql/data are always stored in a Docker volume or mount point. In your case you didn’t use a docker run -v option, but the image is configured to create an anonymous volume in that directory. The volume is not included in docker commit, which is why you’re not seeing it on the rebuilt container. (Also see docker postgres with initial data is not persisted over commits.)

    For editing a configuration file, the easiest thing to do is to store the data on the host system. Create a directory to hold it, and extract the configuration file from the image. (Since the data directory is created by the image’s startup script, you need a slightly longer path to get it out.)

    mkdir pgdata
docker run -d --name pgtmp postgres:9.6.24
docker cp pgtmp:/var/lib/postgresql/data/pg_hba.conf ./pgdata
docker stop pgtmp
docker rm pgtmp

$EDITOR pgdata/pg_hba.conf

    Now when you run the container, provide this data directory as a bind mount. That will inject the configuration file, but also cause the database data to persist over container exits.

    docker run -v "$PWD/pgdata:/var/lib/postgresql/data" -u $(id -u) ... postgres:9.6.24

    Note that this sequence doesn’t use docker exec or "go inside" containers at all, and you haven’t created an image without corresponding source. Everything is run with commands from the host. If you do need to reset the database data, in this setup, it’s just files, and you can rm -rf pgdata, maybe saving the modified configuration file along the way.

    (If I’m reading this configuration change correctly, you’re trying to globally disable passwords and instead allow trust authentication for all inbound connections. That’s not usually a good idea, especially since username/password authentication is standard in every database library I’ve encountered. You probably still want the volume to persist data, but I might not make this change to pg_hba.conf.)

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